I have consulted "The Book" (that is what I call it), page 179 (index attached).
The Holy Grail of castironism, so to speak.
Be back shortly.
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Edit:
Annealing generally reduces the grade level of gray iron one level, ie: if starting with Class 40 gray iron, it will be slightly less strong Class 30 gray iron after the annealing process. Not a problem for model engine castings in my opinion.
There are three typical gray iron annealing processes:
1. Low-temperature (ferritizing) annealing.
2. Medium-temperature (full) annealing.
3. High-temperature (graphitizing) annealing.
Low temperature annealing is between 1,300 F and 1,400 F, at 1 hour per inch of section.
Medium temperature annealing is between 1,450 F and 1,650 F.
High-temperature annealing is between 1,650 F and 1,759 F.
Once a piece of cast iron has been heated, it should be cooled slowly.
Slow cooling is also important for stress relief.
Apparently the medium and high temperature annealing are used when the iron is of higher alloy content, and does not respond to the low-temperature annealing process.
I would suggest starting with the low-temperature annealing process, and try that method first, to see if it improves machinability significantly.
If you don't have a kiln with electronic temperature controls, then I guess you could try the charcoal method that JasonB mentions.
I have heard that if you use a torch of some type to heat a gray iron casting, then it should be heated somewhere between "medium red" and "red", per the attached cast iron color chart (not an exact method, but would probably be accurate enough).
The torch method (depending on exactly what type of torch is used, and how concentrated the flame is) is a good method for annealing just a small area of an overall casting, such as the bosses you were trying to drill.
I guess if I were annealing, I would just anneal the entire piece, just in case there were other hard spots somewhere else in the casting.
And as I mentioned, use slow cooling regardless of which method is used.
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